1. Field of the Invention
The present invention relates to an optical sheet and more particularly to an optical sheet for use in a backlight device or the like in a display device.
2. Description of the Related Art
Conventionally, CRTs (cathode ray tubes) have been commonly used in display devices such as television sets. In recent years, however, flat-panel displays, with the advantages of being space-saving and power-saving, and with increasingly improved viewing angle, contrast, color reproduction and other characteristics, have been coming into wider use. In particular, liquid crystal display devices are replacing CRTs.
In liquid crystal display devices, since liquid crystal itself does not emit light, backlight or external light is shone on the display area formed by the liquid crystal, and the alignment of the liquid crystal is controlled by a drive circuit to control the amount of light passing through the liquid crystal. In this way, liquid crystal display devices display images. Here, the light from the light source such as a backlight needs to be shone evenly on the liquid crystal forming the display area, and to achieve that, optical sheets such as a diffusion sheet and a prism sheet are interposed between the backlight and the liquid crystal. Most such optical sheets are formed of resin, and thus they expand and contract as the ambient temperature varies. Hence, fixing optical sheets in a display device requires a design that accommodates the expansion and contraction of the optical sheets.
Conventionally, optical sheets are fitted into a frame, for example, as shown in FIG. 9. Specifically, at the center of opposite sides of optical sheets 3′, there are formed notches 31′ that are substantially rectangular and arc-shaped at the dead end; correspondingly, at the center of opposite sides of a step 12 in a case (frame) 11 into which the optical sheets 3′ are going to be fitted, there are formed projections 14′ that are similar in shape to but slightly smaller than the notches 31′. The notches 31′ in the optical sheets 3′ fit around the projections 14′ on the case 11, and this permits the optical sheets 3′ to be positioned in place.
Alternatively, as shown in FIG. 10, rectangular slots 41′ are formed in opposite side edges of optical sheets 4′, and correspondingly projections 15′ are formed at the center of the opposite sides of a step 12 in the case 11 into which optical sheets 4′ are going to be fitted. The slots 41′ in the optical sheets 4′ fit around the projections 15′ on the case 11, and this permits the optical sheets 4′ to be positioned in place.
Inconveniently, however, with the former method of positioning the optical sheets 3′ which involves the engagement of the substantially rectangular notches 31′ with the projections 14′, as shown in FIG. 11, the notches 31′ in the optical sheets 3′ may disengage from the projections 14′ due to the thermal contraction of the optical sheets 3′ during use. If the optical sheets 3′ disengage from the projections 14′, the disengaged side of the optical sheets 3′ sinks under gravity, causing the optical sheets 3′ to warp or deform. This causes defects such as uneven display in a display device. With the latter method of positioning the optical sheets 4′, when the slots 41′ are formed in them, or when they receive an external impact, as shown in FIG. 12, a tear may develop from a slot 41′ to a side of the optical sheets 4′.